The invention relates to a process for the recovery of pure aromatics from aromatics-containing hydrocarbon fractions and, in particular, from reformate gasoline, from fully hydrogenated pyrolysis gasoline, from coke-oven light oil refined under pressure or from mixtures thereof, with extractive distillation and recovery of the extracting solvent taking place in one column. The process according to the invention serves to provide aromatics and especially benzene, the alkylated benzene derivatives of the aromatics factions being converted by hydrodealkylation to give benzene or lower alkylated benzene derivatives. Depending on the design of the process high-purity products can be produced. By a novel process configuration it is possible to achieve a significant saving in expensive hydrogen used for the hydrodealkylation and in technical equipment. The invention also relates to an apparatus which serves to carry out this process.
Aromatics, particularly benzene, toluene and xylenes are important feedstocks used in the chemical industry, especially for the production of plastics and man-made fibres. In addition, aromatics are used to boost the octane number of Otto fuel. For the chemical industry application it is favourable to provide the required aromatics in the form of pure benzene as the demand for this chemical is especially high.
To recover benzene by hydrodealkylation, reformate gasoline, fully hydrogenated pyrolysis gasoline, coke-oven light oil refined under pressure or mixtures thereof may be used as hydrocarbon mixtures rich in aromatics. These feed products or mixtures thereof are hereinafter referred to as gasoline fractions. The used gasoline fractions contain major amounts of benzene derivatives, especially alkylaromatics. These can be converted by thermal or catalytic hydrogenation to give benzene, in which the alkyl substituents of the benzene are released and fully hydrogenated by reaction with hydrogen. This process of hydrodealkylation (“HDA”) serves to obtain the required benzene. Apart from the required benzene, the alkyl substituents are obtained in the form of alkanes.
The reformate gasoline is a benzene rich in aromatics, which is produced by reforming and especially catalytic reforming of naphtha. During the reforming process, the alkanes and cycloalkanes contained in the petroleum or crude oil are subject to isomerizations, rearrangements, cyclizations, dehydrogenations and similar reactions. The aromatics-rich reformate gasoline obtained from catalytic reforming serves as an important feedstock in the recovery of aromatics.
The fully hydrogenated pyrolysis gasoline is a gasoline rich in aromatics which is obtained from steam cracking of hydrocarbons. Steam cracking of hydrocarbons mainly serves to generate lower olefins, especially ethene. Depending on the boiling range of the hydrocarbon mixture used for steam cracking, a large amount of a by-product rich in aromatics is obtained, the so-called pyrolysis gasoline, which, for further processing, is yet to be freed from unsaturated compounds and hetero-atoms (sulphur, nitrogen, oxygen) in various hydrogenation steps (selective hydrogenation, full hydrogenation). The product obtained from the hydrogenation steps is an aromatics-rich fraction which is referred to as a fully hydrogenated pyrolysis gasoline.
The coke-oven light oil refined under pressure is also a product rich in aromatics. Coke-oven crude light oil is obtained from the coking of coal. Similar to the pyrolysis gasoline, this coke-oven crude light oil still contains unsaturated compounds and hetero-atomic compounds apart from aromatics. Similar to the treatment of the pyrolysis gasoline, the coke-oven crude light oil is also hydrogenated to convert the unsaturated compounds as well as the hetero-atomic compounds. One of the products of this conversion is an aromatics-rich product which is free of unsaturated compounds and hetero-atomic compounds and can be used in a subsequent process. This aromatics-rich product is also referred to as coke-oven light oil refined under pressure.
The aromatics-rich hydrocarbon mixtures used may contain major amounts of non-aromatic compounds such as especially paraffins and naphthenes or olefins which are also hydrogenated in the hydrodealkylation giving alkane as hydrogenation product. If the hydrogenation is continued, methane is obtained from the cracking of alkanes. Depending on the stoichiometric amount of hydrogen used, the hydrodealkylation can also give lower alkylated benzene derivatives which have not been dealkylated completely to benzene.
The presence of large amounts of non-aromatic compounds in the feed stream to the hydrodealkylation requires an adequately large amount of hydrogen to carry out the reaction since hydrogen is not only required to dealkylate the alkylaromatics by hydrogenation to give benzene or lower alkylated benzene derivatives but also to decompose the longer chain non-aromatic compounds to short chain non-aromatic compounds. If the longer chain compounds are decomposed completely, methane is obtained. Therefore it is an aim of the invention to provide a process for the hydrodealkylation which minimises the use of hydrogen.
Apart from the required benzene, the hydrodealkylation also gives a mixture of short-chain paraffins or methane. The amount of gas generated by the dealkylation of aromatics cannot be influenced by the process according to the invention. It is, however, also the aim of this invention to reduce the amount of gas generated by the hydrogenation of the non-aromatic compounds also entrained in the feed stream.
To achieve these aims it is advantageous to first purify the hydrocarbon fraction to be submitted to hydrodealkylation in an extractive distillation. By carrying out the extractive distillation it is possible to separate a major part of the non-aromatic compounds already. The aromatics concentrate obtained from the extractive distillation can then be passed to a hydrodealkylation. As no hydrogen is consumed by the chain-shortening hydrogenation of the non-aromatic compounds which have already been removed from the starting mixture by an extractive distillation, a reduction in the amount of hydrogen consumed by the hydrodealkylation is achieved. In addition, the substance mixture contains lower portions of gaseous paraffins owing to the upstream installation of the extractive distillation after the hydrodealkylation. In this way it is possible to design the whole process for a lower gas flow rate which is of advantageous effect.
EP 792928 B1 describes a process for the recovery of pure aromatics from reformate gasoline. The teaching discloses a process in which, in a first process step, a reformate cut with aromatics of a selected carbon number or with aromatics of several selected carbon numbers Cx, Cy is recovered from the reformate gasoline by fractionating distillation and the aromatics cut is hydrogenated selectively via a hydrogenating catalyst in a second process step, and, in a third process step, the selectively hydrogenated and aromatics-containing products from the second process step are then separated into aromatics and non-aromatic compounds by extractive distillation and/or liquid-liquid extraction. The hydrogenation conditions of the second process step are adjusted such that non-aromatic unsaturated hydrocarbons such as especially olefins, diolefins and triolefins are hydrogenated as well and conjugated diolefins and triolefins are hydrogenated as fully as possible.
DE 1568940 A1 describes a process for the separation of aromatics from hydrocarbon mixtures of any aromatics content, which may contain paraffins, cycloparaffins, olefins, diolefins and organic sulphur compounds as non-aromatic components, by extractive distillation. In the extractive distillation, especially N-substituted morpholines are used the substituents of which do not have more than seven C atoms. The aromatics fraction obtained can be submitted to a post-treatment, reference being made to a downstream sulphuric acid washing or a clay treatment. The impurities in the extracting agent, which especially consist of unsaturated hydrocarbons, gather in the bottom phase of the column and can be separated by bottom phase dehydrogenation and extractive distillation.
DE 10019196 C1 describes a process for the recovery of a high-purity aromatics product consisting of benzene and toluene or toluene and xylene from a close-boiling or azeotropic boiling intermediate product containing non-aromatic compounds and an apparatus for carrying out the process. The extractive distillation of non-aromatic compounds and the recovery of the extracting agent are performed in a single column consisting of a column main section comprising two parallel compartments, a rectifying section above the column main section, a stripping section below the column main section and a bottom with associated bottom heating. The starting gasoline is previously separated into at least two fractions by way of distillation, one of which is an aromatics fraction containing higher boiling non-aromatic compounds and a second is an aromatics fraction containing lower boiling non-aromatic compounds. The two fractions are introduced at separate feed points of the compartment of the column main section which is open at the upper and the lower end, the higher boiling aromatics fraction being introduced above the lower boiling aromatics fraction.
It is therefore the aim to provide a process in which an aromatics-rich starting gasoline is first depleted from the non-aromatic hydrocarbons by extractive distillation and the resulting aromatics concentrate is dealkylated by hydrogenation and converted into benzene or dealkylated aromatics as a product. After the hydrodealkylation the obtained aromatics concentrate should have such purity level that simple processing such as flashing, rectifying or both is enough to purify it. Depending on the design it should also be possible to add another hydrogenation treatment between the process steps of extractive distillation and hydrodealkylation in order to remove the residual olefins and polyunsaturated non-aromatic hydrocarbons.
Suitable feed gasolines for the extractive distillation, which have an adequately high content of aromatics, are, for instance, coke-oven light oil refined under pressure, fully hydrogenated pyrolysis gasoline or reformate gasoline which is usually obtained in large quantities in refineries. Depending on the starting gasoline it may be distilled into fractions before being used to increase the content in aromatics. The inventive apparatus should also be space-saving and facilitate reduced investment and operating costs.